Intruder alarm system

ABSTRACT

In an intruder detector a beam of electro-magnetic energy pulses is directed across a space to be protected and onto a photodetector, the electrical pulse output of the photodetector being utilized to discharge a capacitor in one pulse. When the charge on the capacitor exceeds a threshold amount an alarm is thus sounded in response to the absence for a time of electromagnetic energy pulses falling on the detector. The photodetector is preferably a phototransistor with its base electrode biased so that the phototransistor doubles as an amplifier. A timing circuit is provided to continue to sound an alarm for the same preset period of time after each interruption of the electromagnetic energy beam pulses. The intruder alarm receiver of the electro-magnetic energy pulses is either selfcontained or operated in conjunction with a remote master control unit, the two units being connected so that an alarm will sound at the master control unit if a wire connecting the two is broken, all without the necessity for extra continuity checking conductors.

United States Patent [1 1 Cho [ Oct. 21, 1975 [5 INTRUDER ALARM SYSTEM [75] Inventor: Fan Pong Cho, Kowloon, Hong Kong [73] Assignee: Franciscan Enterprises, Inc.,

Sunnyvale, Calif.

[22] Filed: Dec. 17, 1973 [21] Appl. No.: 425,195

[52] US. Cl 340/258 B; 340/276; 340/409 [51] Int. Cl. G08B 13/18 [58] Field of Search 340/258 B, 276, 409;

[56 References Cited UNITED STATES PATENTS 3,465,158 9/1969 Foerster 307/311 3,605,082 9/1971 Matthews 340/258 B 3,752,978 8/1973 Kahl, Jr. et al.. 340/276 3,820,102 6/1974 Schubert 340/409 OTHER PUBLICATIONS BE AM PULSE DETECTOR PULSE DELAY PULSE l CONVERTER AMPLIFIER Primary ExaminerGlen R, Swann, Ill Attorney, Agent, or FirmLimbach, Limbach & Sutton [57] ABSTRACT In an intruder detector a beam of electro-magnetic energy pulses is directed across a space to be protected and onto a photodetector, the electrical pulse output of the photodetector being utilized to discharge a capacitor in one pulse. When the charge on the capacitor exceeds a threshold amount an alarm is thus sounded in response to the absence for a time of electromagnetic energy pulses falling on the-detector. The photodetector. is preferably a phototransistor with its base electrode biased so that the phototransistor doubles as an amplifier. A timing circuit is provided to continue to sound an alarm for the same preset period of time after each interruption of the electromagnetic energy beam pulses. The intruder alarm receiver of the electro-magnetic energy pulses is either selfcontained or operated in conjunction with a remote master control unit, the two units being connected so that an alarm will sound at the master control unit if a wire connecting the two is broken, all without the necessity for extra continuity checking conductors.

MULTI v: BRATOR US. Patent Oct.21, 1975 Sheet10f3 3,914,753

.wtwtsU L QU O U.S. Patent Oct. 21, 1975 Sheet 2 of3 3,914,753

Jig. 4-

U.S. Patent oat. 21, 1975 Sheet 3 of 3 3,914,753

INTRUDER ALARM SYSTEM BACKGROUND OF THE INVENTION The detection of an intruding person or object in a building or in an outdoors area along a line of sight path is desired in a great number of circumstances. An in truder in a line of sight path is detected, according to one system, by the generation of a beam of invisible electro-magnetic energy pulses, usually in the infrared frequency range. A transmitter of such pulses is positioned at one location and the beam of light is directed along a line of sight to a receiver at a distant location. The receiver is designed to sound an alarm or give some other indication of the presence of the intruder when pulses of a predetermined time interval are not received at the receiver. One example of this is described in a magazine EDN/EEE of Mar. 15, 1972, page 62. U.S. Pat. Nos. 3,534,351 Harden and 3,605,082 Matthews also describe such systems. However, the systems of prior art suffer from certain disadvantages of high power requirements, a high probability of falsely triggering their alarm and an ability of a intruder to disable the alarm system.

Accordingly, it is an object of the present invention to provide an intruder alarm system with a reduced power consumption and a minimum number of circuit components.

It is another object of the present invention to provide an intruder alarm with a reduced probability of being falsely triggered.

It is yet another object of the present invention to provide an intruder alarm system with a separately spaced units that cannnot be unknowningly disabled by communicating wires therebetween being cut.

It is still another object of the present invention to provide an intruder alarm system which is compatible with commercially available door and window intruder sensors (in addition to optical path beam receivers) and also with fire sensors in order to provide complete integrated intruder and fire alarm system.

SUMMARY OF THE INVENTION An electro-magnetic energy pulse receiver is provided with a phototransistor with its base biased so that the operation of a phototransistor is in a region of higher gain than if the base terminal is left unattached as is the usual case with existing devices and techniques. That is, light pulses received by the phototransistor appear as electrical pulses at its collector output with greater magnitude, possibly by a factor of 10, over the magnitude of the electrical pulse output for the same light pulse intensity with the base of the phototransistor unbiased.

A transistor amplifier of the single polarity output pulses of the phototransistor as its base biased at a very low level so that the transistor amplifier is operating on a non-linear portion of its characteristic curve. That is, the bias is set so that the amplification of the single polarity pulses from the phototransistoris much greater then it would be for pulses with an opposite polarity. The result is that the power consumption by this amplifier stage is reduced since the collector current of this transistor amplifier is at a very low level in between pulses. Reduction of power comsumption is extremely important in intruder alarm systems since it is generally desired to operate then from batteries so that they are not dependent on the continuity of commercial power.

In order to detect when an intruder has passed through the electro-magnetic energy pulse beam, the amplified electrical pulses from the phototransistor are applied to a switching transistor which discharges a first storage capacitor for the duration of each received pulse. This capacitor is normally charged by a low current through a resistor connected in series with the capacitor and the direct current voltage source. When a few pulses are blocked by an intruder passing through the electro-magnetic energy beam, the voltage across the capacitor rises until it reaches a set threshold value which is detected to initiate an alarm. The switching transistor which discharges the storage capacitor during each received light pulse is connected directly across the capacitor without any other impedance element in series therewith and is designed to discharge the capacitor substantially completely upon the receipt of one pulse. This has the advantage that the voltage is not permitted to build up across the capacitor when a few random pulses are not received because of dust, insects flying through the beam, and similar reasons. Thus, false triggering of the alarm from such causes is significantly reduced.

Once the voltage in this first storage capacitor exceeds the detected threshold level, an alarm is initiated by appropriate triggering circuits. The duration of the alarm, usually of an audio type, is made to be independent of the length of the time that the electro-magnetic energy beam is broken so that the alarm may be set to sound for a sufficient time to alert someone nearby. This is accomplished by a second capacitor that is immediately charged when the alarm is initiated while a threshold trigger circuit monitors the voltage across the capacitor. This voltage is permitted to decay at a controlled rate toturn off the alarm when the capacitor voltage reaches a preset minimum. The length of alarm sounding is controlled by the time constant of the discharge path of the capacitor and by the lower threshold value which is sensed to turn off the alarm activating circuits.

For a more complete alarm system and for other reasons, it is often desirable to locate the alarm a distance from the electro-magnetic pulse beam receiver. The receiver is connected to a remote master control unit including the alarm by a number of conductors necessary to communicate an alarm initiating signal and power. If any one of the communicating conductors is broken by an intruder, the alarm system will not be disabled but rather the alarm receives an initiating signal in the same manner as it is turned on by an intruder breaking the electro-magnetic energy pulse beam. This is accomplished in part by sending a DC signal between the receiver and master control units for activating the alarm having a level that is the same as will be seen by the master control unit if one of the signal communicating conductors is cut. In a specific form, this is accomplished by including as an output element of the receiver a semi-conductor element which is rendered normally conductive expect when the alarm is to be sounded. A load element of this output semi-conductor device is positioned at the input of the master control line are broken by an intruder, the alarm is sounded. No additional wires or other physical paraphernalia are required to detect the breaking of the communicating conductor between receiver and master control units. Other objects, advantages and features of the various aspects of the present invention will become apparent from the following description of a preferred embodiment thereof, whichshould be taken in conjunction with the accompanying drawings. In the preferred embodiment of the invention to be described, a infrared beam of pulses is utilized but it will, of course, be understood that visible light of other kinds of radiation may be directed across an area to be protected as well. Also, it will be appreciated from the following description that many of the circuit concepts and features have general application in other specific intruder alarms.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring initially to FIG. 1, a light beam transmitter includes a power supply 11 which may be any convenient source of energy such as a battery, house power, etc. A pulse generator 12 receives power from the supply 11 and drives a light emitting diode 13 with a train of periodically recurring narrow width pulses. For a usual application of an intruder alarm, it is desires that the light beam be invisible. With this requirement, the diode 13 is a standard gallium arsenide diode which emits infra-red radiation of a frequency just outside the visible spectrum. The radiation emitted by the diode 13 is in the form of narrow width pulses wherein the pulse generator 12 essentially and repetitively turns the diode 13 on and off.

The pulsed light beam generator also includes a lens 14 which produces a collimated light beam 15 which is directed at a light gathering lens 16 that is part of a receiver unit 17. The receiver 17 includes an alarm 18 which maybe any indicating device andis described in the example herein as an audio alarm. The indicating device 18 is activated by the receiver 17 when pulses within a predetermined time interval are not consecw tively received by the receiver 17. A series of pulses would be missing from the light beam when an intruder passes through that beam. It is this condition of an intruder being present that is desired to be detected and indicated by the alarm 18.

The receiver 17 is shown to be completely selfcontained including an alarm. It may be desirable, however, in many circumstances to have a master control unit 19 interconnected by cable 20 to the receiver 17 and remotely positioned therefrom. In this case, the alarm l8'will be deactivated with a similar alarm' 21 of the master control unit 19 being activated in its place. The receiver 17 is described in detail with respect to FIG. 2 and the master control unit 19 is described in detail with respect to FIG. 5. The master control unit, as described hereinafter, makes possible the connection of a number of receivers of the type of receiver 17 and also permits connection of available fire, door and window opening sensors. When the master unit 19 is utilized, not only is the alarm function transferred to the master unit from the receiver 17 but also the power supply is transferred thereto. By maintaining the power and alarm in the same physical unit, the chances of disabling the alarm circuits by cutting the wires interconnecting the units is rendered virtually impossible, as described hereinafter.

Referring to FIG. 2, the details of the receiver unit 17 in FIG. 1 are described. A phototransistor 113 of a commonly available type is connected with its emitter connected directly to a common potential bus 22. The collector of the transistor 113 is connected through a series resistor 114 to a voltage bus 23. A coupling capacitor is connected to the collector of the phototransistor 1 l3 and presents at a terminal 25 a time varying electrical signal in accordance with the time varying optical signal impressed upon the phototransistor 113 through its lens 16. I

Instead of using the phototransistor 113 as a photodiode as has previously been done by omitting connection to its base lead, the circuit of FIG. 2 provides a resistor 112 between the voltage supply line 23 and the base lead of the phototransistor 113 for biasing the phototransistor into a region of operation that permits the phototransistor 113 to double as an amplifier. That is, the current supplied to the base through the resistor 112 increases the steady state collector current I that would exist in the phototransistor 113 when the base lead remains unconnected to other circuit components. Referring briefly to FIG. 3, a typical curve for a transistor gain versus collector current I is shown. Without the base lead of the phototransistor 113 connected to a biasing source, the collector current would be a very low value, such as in the region of 94 as shown in FIG. 3. By biasing the base of the phototransistor 113 in a manner illustrated in FIG. 2, this steady state (no signal) current may be increased to a value such as that indicated at 96 of FIG. 3. As a result, the overall gain of the phototransistor 113 is increased. A capacitor 111 is connected between the common point and the base lead of the phototransistor 113 as well in order to reduce static pick-up by the phototransistor 113.

The electrical pulse signal at the terminal 25 is then applied to a pulse amplifier whose primary element is a transistor 117. The emitter of this transistor is connected directly to the voltage supply line 23 whileits collector is connected through a series resistor 118 to the common line 22. A biasing resistor 116 is connected between the base lead of the amplifying transistor 117 and the common line 22 in order to increase the steady state (no signal) collector current in the trannsistor 117 a small amount. Therefore, a pulse signal applied to the base lead of the transistor 1 17 is amplified and coupled through a capacitor 119 to an output terminal 27. In order to conserve battery power, the transistor 117 is biased by its base resistor 116 to anon-linear region of its gain characteristic curve of the type illustrated inFIG. 3. This is, the collector current I for the transistor 117 with no signal applied to its base is set by the value of its biasing resistor 116 to be on a steeply sloping portion of its gain curve as illustrated in FIG. 3, such as in the region of 98. It is usually the practice to bias an amplifying transistor so that its signal operates in the flat region 99 of its characteristic curve (FIG. 3) but in this case since we are dealing with single polarity pulses, the zero or no pulse collector may be placed in the region such as 98 (FIG. 3). This significantly reduces the current drain on the power source between the pulses.

The amplified electrical pulses are then applied from the terminal 27 to the base of a transistor 121 which is biased by a base resistor 120 to be normally nonconductive. As soon as enough current flows through the base resistor 120 from the signal impressed at the terminal 27, usually enough to create'a voltage drop of about 0.6 volts which is the forward voltage drop across a semi-conductor junction of the transistor 121, transistor 121 becomes conductive. Therefore, as a pulse rises from zero past this lower threshold value at the terminal 27, the transistor 121 is rendered conductive to draw large current to discharge a first storage capacitor 123. This first storage capacitor 123 is connected across the collector and emitter of the transistor 121 directly without any resistance or any impedance elements connected in series therewith so as to facilitate this discharge. The first storage capacitor 123 normally is being slowly charged through the resistor 122 since this series circuit combination is always connected between the voltage supply line 23 and the common line 22. When the transistor 121 is rendered conductive by an input pulse, however, the capacitor 123 is practically shorted out and is thus rapidly discharged. The typical impedance of a transistor 121 in its conductive state is less than 100 ohms, so the capacitor 123 is rapidly discharged. The width of each of the pulses are made sufficient so that the transistor 121 is on long enough for particular circuit components chosen to completely or substantially completely discharge the capacitor 123 for each pulse. A buffer transistor 124 is provided to prevent current drain on the capacitor 123. The collector of the transistor 124 is connected directly to the voltage supply line 23, its base to one side of the capacitor 123 and its emitter through series resistors 125 and 126 to the common line 22. The junction between the resistors 125 and 126 in the emitter circuit of the transistor 124 provide the output voltage at a terminal 28 which is subsequently utilized.

Referring to FIG. 4, some voltage waveforms of the circuits described up until this point with respect to FIG. 2 may be observed. FIG. 4(a) shows the light pulses being generated by the transmitter unit and directed across a space to be protected onto the phototransistor 113. If there is an interruption or a blockage of this beam temporarily, a few pulses will not reach the phototransistor 113, as indicated in FIG. 4(b). The pulse train shown in FIG. 4(b) is that received by the phototransistor 113 when an intruder passes through the beam 15.

The voltage across the capacitor 123 is illustrated in FIG. 4(c). The voltage at the terminal 28 has substantially the same waveform but a different magnitude. From FIG. 4(0) it can be seen how the voltage builds up across the capacitor 123 during the time that pulses are not received. As soon as a pulse 29 is received after the period of interruption, the capacitor 123 iscompletely discharged by the transistor 121. It is this rising voltage which is monitored by subsequent circuits of FIG. 2 to sound the alarm 18.

An amplifying transistor 127 is unbiased and receives a signal at its base from the terminal 28. The emitter of the transistor 127- is connected directly to the common line 22 while its collector is connected through series resistors 128 and 129 to the voltage supply line 23. The signal at a terminal '30 is derived from the junction between the series connected resistors 128 and 129. This signal is applied to a base electrode of a transistor which is also unbiased and has its emitter connected directly to the voltage supply line 23 and its collector connected through a switch 153 and a second storage capacitor 151 to the common line 22. A diode 152 is connected across the transistor 150 and switch 153 between the capacitor 151 and the voltage supply line 23. A load resistor l54is connected between the collector of the transistor 150 and the common line 22.

The transistors 127 and 150 are normally in an off condition until the voltage builds up across the capacitor 123 to a certain threshold level such as the level 31 indicated in FIG. 4(c). When this happens, a terminal 32 at the output of the alarm timer circuit becomes connected directly to the voltage supply line 23 through the transistor 150 which is now in its on state. The exact threshold level 31 is determined by the characteristics of the unbiased transistors 127 and 150 as well as the relative values of the voltage dividers 125, 126 and 128, 129. This threshold level 31 is set to require the omission of light beam pulses for a set period time (such as the time for two pulses) so that the alarm will be sounded only when the beam is completely broken by an intruder and will not be accidently sounded by a bug flying through the beam, dust etc. which may block only one pulse.

FIG. 4(d) shows the voltage at the collector of the transistor 127 with respect to the common line 22. The turning on of transistor 127 at the threshold level 31 drops the voltage of its collector to substantially zero volts, that potential of the common line 22. This same signal, but in different magnitudes, is applied to the base of the transistor 150 and thus the transistor 150 remains turned on for the same length of time until the capacitor 123 becomes discharged. When the transistor 150 is turned on, the capacitor 151 is rapidly charged to the full voltage since it is effectively placed directly between the voltage supply line 23 and the common line 22. When the transistor 150 is again turned off in response to the first storage capacitor 123 being discharged by a received light pulse, the capacitor 151 slowly discharges through the resistor 154 at a rate determined by the time constant of the resistor 154 and capacitor 151 combination. A voltage waveform at the output terminal 32 of the alarm timer is shown in FIG. 4(e) wherein a full voltage is impressed for a time across the second storage capacitor 151 while the transistor 150 is on, and then the voltage across the capacitor 151 begins to decay as it is discharged through the resistor 154. This signal at the terminal 32 permits the alarm 18 to be sounded for a period of time that is set independently of the time that the light beam is broken by an intruder.

A trigger circuit receives this signal of FIG. 4(e) at the terminal 32 and provides at an output terminal 33 a control signal as illustrated in FIG. 40) which is initiated when the capacitor 151 is first charged above a threshold level 34 of FIG. 4(e). The control signal ends,

as shown in FIG. 4(f) when the second storage capacitor 151 discharges to a-second threshold level 35. This control signal at the terminal 33 as illustrated in FIG. 4(f) can then be used to operate any kind of alarm system but is shown in FIG. 2 to turn on a multivibrator circuit which gives an audio signal at an output terminal 36. The audio signal drives a loudspeaker or other audio alarm 18 through power amplifier transistors 173 and 174. The length of time, therefore, that the alarm 18 is sounded depends upon the value of the second threshold level 35 that is set by the trigger circuit and also the time constant of the capacitor 151 and resistor 154. If it is desired that the alarm 18 only sound during the period of interruption of the light beam 15, the switch 153 is opened and the capacitor 151 is not operative, thereby not maintaining the sounding of the alarm 18. The switch 153 is convenient in case an occupant of a dwelling desires to turn off the alarm after the beam has been broken or wishes to check to see if the beam is still broken at any particular instant when the alarm is sounding. The diode 152 discharges the capacitor 151 very rapidly if power is lost in the power line 23, such as by turning off the main supply power.

Transistors 157 and 160, along with resistors 155, 156, 158, 159 and 161 form the threshold circuit between the terminals 32 and 33 which senses the voltage on the collector electrode of the transistor 150 through the terminal 32. Positive feedback within the trigger circuit through the resistor 156 causes the threshold circuit to have an upper threshold 34 and a lower threshold 35 (FIG. 4(e)). When the transistor 150 becomes conductive, transistors 157 and 160 are switched to their conductive state. When the voltage at the terminal 32 drops below the second threshold level 35, the transistors 157 and 160 switch back to their non-conductive state. This trigger circuit is similar in philosophy to the well known Schmidt circuit.

An ordinary multivibrator circuit is established between the terminals 33 and 36 and includes transistors 166 and 170, capacitors 165 and 169 and resistors 164, 167, I68, I71 and 172. The control signal at the terminal 33 operates the multivibrator circuit through bias resistors 167 and 168. Thus, an audio signal is generated at its output terminal 36 by the multivibrator when the control signal at the input 33 is up for a period as shown in FIG. 4(f). A diode 163 and capacitor 162 suppress the signal generated by the multivibrator and keep it isolated from the rest of the circuit.

The receiver may be provided with an internal voltage source 176 or may receive its voltage through a line 37 from the master control device 19 at a receiver terminal 38. A switch 175 is provided in the receiver for selecting whether an internal or an external voltage source is utilized. Since it is desired to have the voltage source of an intruder alarm system in the same unit as its alarm unit, a switch 177 is provided to be ganged with the switch 175 so that the alarm 18 is rendered inoperative when a master unit voltage source is utilized. The switches 175 and 177 are shown in FIG. 2 in their positions wherein the receiver is utilized without a master unit 19. These switches would be thrown to their opposite positions if a master unit 19 is utilized.

The receiver is also provided with a common terminal 39 for communicating through a line 40 to the master control unit 19. Also, a terminal 41 is provided for sending a signal through a line 42 to a master control unit when an intruder has blocked the pulsed light beam. Only three lines 37, 40 and 42 are therefore required as the interconnection 20 of FIG. 1 between the receivers 17 and master control unit 19. An inverting transistor 132 has its collector connected directly to the output terminal 41 of the receiver unit and its emitter terminal connected directly to the common line 22. The base terminal of the transistor 132 is connected between a voltage dividing circuit of series resistors and 131 which are connected between the collector of the transistor 127 and the common line 22. It will be noted that there is no load resistor associated with the inverter transistor 132 in the receiver unit of FIG. 2 but rather it is included in the master control unit, as explained hereinafter with respect to FIG. 5. When the receiver unit is interconnected with the master control unit, the voltage at the output 41 increases from a substantially zero level, as the transistor 132 becomes nonconductive during the time that an intruder is blocking the light beam 15.

Referring to FIG. 5, a master control unit 19 having the alarm 21 is illustrated in detail. Since the alarm timer, trigger circuit, multivibrator and audio alarm circuits of the master unit illustrated in FIG. 5 are substantially the same as those described above with respect to the receiver of FIG. 2, these circuit blocks will not be described again but rather their components are identified with the same numbers that were used in FIG. 2 except that the reference numbers utilized in FIG. 5 include a prime mark following the corresponding number. It will also be noted that these circuits are operating with different polarities for convenience but that they are in principal the same as those described above with respect to the receiver of FIG. 2.

Referring to the input circuits of the master unit of FIG. 5, a terminal 41' connects the collector of the receiver inverter transistor 132 through the line 42 to a collector load resistor 200 which is also connected to a voltage supply line 43 of the master control unit. A terminal 38' is provided for connection with a voltage supply line 37 which goes to the receiver of FIG. 2. Similarly, a terminal 39 is provided for connecting with the common line 40 that extends between the receiver of FIG. 2 and the master control unit of FIG. 5. The signal at the collector of the inverter 132 (FIG. 2) is applied to base terminal of a transistor 223 through series connected diode 201 and resistor 221. A capacitor 222 extending between the base of the transistor 223 and a common line 44 of the master control unit, in conjunction with the resistors 220 and 221, dampens the circuit to eliminate any high frequency noise that may exist as a result of being induced in the lines connecting the master control unit with the remote receiver. When an intruder blocks the pulsed light beam 15, the inverter transistor 132 is turned off and this causes the voltage at the terminal 41' of the master control unit to suddenly go up as illustrated in FIG. 4(g). While the inverter transistor 132 of the receiver is turned on when no intruder is in the path of the light beam 15, the voltage at the terminal 41 is substantially zero which renders the transistors 223 of FIG. 5 nonconductive. When either the light beam 15 is broken by an intruder for sufficient time to turn off the transistor 132 or the line 42 is cut, the transistor 223 is rendered conductive which results in sounding the alarm 21.

The collector of the transistor 223 is connected directly with the voltage supply line 43 while its emitter is connected through series resistors 224 and 225 to a common line 44. A point between the resistors 224 and 225 is connected to a base of a transistor 226 which has its emitter connected directly to the'common line 44 and its collector connected through series load resistors 227 and 228 to the voltage supply line 43. When the transistor 223 becomes conductive by either an intruder breaking the light beam 15 or the line 42 being cut, the transistor 226 also becomes conductive and applies its collector voltage through a series resistor 232 to a base connection of the transistor 150 as part of an alarm timer. The alarm timer shown in FIG. operates in the same way as described in detail with respect to the receiver circuits of FIG. 2.

It will be noted that if the common conductor 40 interconnecting the receiver of FIG. 2 in the master control unit of FIG. 5 is broken, or if the power supply conductor 37 is severed by someone attempting to disable the alarm, the inverting transistor 132 will become nonconductive and this will turn on the transistor 223 of the master control unit with a resulting sounding of the alarm. This circuitry is an extremely simple way of providing a fail-safe system.

This technique also has the advantage that a plurality of receivers such as that illustrated in FIG. 2 may be connected with a single master control unit of FIG. 5. Referring to FIG. 5, a second input 41" may be connected to an output terminal 41 of a second receiver. The input circuit of FIG. 5 includes a resistor 202 and diode 203 which are counterparts of the first input circuit elements 200 and 201. If a remote receiver unit is not, however, connected to the terminal 41" then a jumper 46 must connect the terminal 41" to the common terminal 39' in order to prevent constant sounding of the alarm 21. For fire protection and for detecting when a door or window is opened, normally closed detectors may be connected to the master unit of FIG. 5 to sound an alarm when these sensors detect a condition for which they are designed. Other inputs may be added to the master unit of FIG. 5 as desired. A separate terminal or a plurality of similarly connected ter minals 47 are attached to the junction between the series load resistors 227 and 228 of the transistor 226. The terminal 47 is provided for connection with normally open circuit sensors. A closing of the sensor circuit with respect to the common terminal 39' causes the alarm 21 to sound. Therefore, the master unit of FIG. 5 is very flexible.

Referring to FIG. 6, a remote alarm 48 is shown very generally having input terminals 49, 50, 51 and 52. These terminals are designed for connection by cables to terminals 53, 54, 55 and 56, respectively, of the master control unit of FIG. 5. The inverting transistors 132' of FIG. 5 operates in the same manner as described with respect to the inverter 132 of FIG. 2. There is a fail-safe interconnection between the master control unit and the remote alarm unit 48. A battery source 57 may be provided in the remote alarm unit 48 for driving both the master control unit of FIG. 5 and the remote receiver of FIG. 2, if desired. Altemately, a voltage source 277 may be applied across the terminals 54 and 56 of the master control unit if the remote alarm unit of FIG. 6 is not utilized or if a power source at the master is desired for some reason. Generally, however, the voltage source should be in the same unit as the alarm, this being the remote alarm unit 48 when it is utilized. In either case, a switch 276 of the master unit of FIG.

5 turns on the power to the master unit (FIG. 5) and its associated receiver unit.

The terminals 53, 54 and 56 are counterparts, respectively, of terminals 41, 39 and 38 of the receiver of 5 FIG. 2. The fourth terminal 55 which is provided in the master of FIG. 5 provides an additional function of enabling a remote alarm unit 48 of FIG. 6 if it is used when power is applied to the master unit. If the master unit of FIG. 5 is turned off a remote alarm in the unit 48 should be deactivated or it will continuously sound. Therefore, a signal is developed at the terminal 55 from a collector terminal of a transistor 264 which tells the remote unit 48 when power is applied to the master control unit and thus enables operation of the alarm unit 48. A second transistor 262 has its base connected through a series resistance 258 to the voltage supply line 43. The transistor 264, with its emitter connected directly to the common line 44, is biased through a resistor 263 from the voltage input terminal 56.

It will be understood that the various aspects of the present invention have been described with respect to a specific intruder alarm system but it will be understood that the invention is entitled to protection within the full scope of the appended claims.

I claim:

1. An electronic circuit for detecting the existence of an object in a space through which an electro-magnetic radiation beam passes, comprising:

a phototransistor having emitter, collector and base electrodes,

means for directing electro-magnetic radiation onto said phototransistor, whereby said phototransistor may be exposed to said electro-magnetic radiation beam,

said collector and emitter electrodes being connected in series with a load resistor across a two terminal voltage supply,

means connected between said voltage supply and said base electrode of the phototransistor for providing a bias that increases the zero radiation signal collector current at least several times a normal collector current that exists when the base terminal remains unconnected, said bias further set for said phototransistor to operate in a non-linear gain region with a collector bias current below that which causes the phototransistor to operate with linear gain characteristics,

an object indicating device having two states, one

state indicating an object present and another state indicating an object not to be present, and

means responsive to the voltage drop across said load resistor for driving said indicating device so that it is in one of its said states when said phototransistor is receiving electromagnetic energy and another state when that electro-magentic energy has been interrupted for a selected period of time.

2. An electronic circuit according to claim 1 wherein said indicating device driving means comprises a nonlinear semi-conductor amplifier element that is biased to operate in a range of its non-linearity with its steady state current at a very low value, whereby said amplifier draws very little current when no signal is impressed thereon.

3. The electronic circuit according to claim 1 wherein said indicating device means comprises:

a direct current voltage supply source,

a storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor,

means receiving said voltage across said load resistor for establishing substantially a short circuit across said capacitor means in response to electromagnetic energy striking the phototransistor, whereby said storage capacitor is immediately discharged by a pulse of electro-magnetic energy of adequate intensity striking the phototransistor, and

means monitoring the voltage level of said storage capacitor for initiating a change of state of said intruder indicating device when said capacitor voltage increases to a predetermined level.

4. The electronic circuit according to claim 3 wherein the object detecting device state change initiating means includes means for instantaneously charging a second storage capacitor simultaneously with a change of state of such indicating device,

means for slowly discharging said second capacitor,

means monitoring the voltage across said second storage capacitor for reversing the change of state of said indicating device when the second capacitor discharges down to a predetermined voltage level.

5. The electronic circuit according to claim 1 which additionally comprises a pulsed source of electromagnetic radiation adapted to form said radiation into a beam that may be directed at said phototransistor, said pulses all having the same polarity with substantially no electro-magnetic energy intensity in the period between pulses.

6. The circuit of claim 1, wherein said bias providing 3 means is a resistance connected between said base electrode and one of the terminals of said voltage supply, and further wherein said circuit additionally comprises a capacitance connected between said base electrode and the other of said voltage terminals.

7. The circuit of claim 1, wherein said indicating device driving means includes a capacitive coupling between said indicating device and a point of connection of said load resistor and an electrode of said phototransistor.

8. An electronic circuit for detecting the existence of an object in a space through which a time amplitude varying electro-magentic energy beam passes, comprising:

a voltage supply source,

a storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor,

means including a photodetector for producing a time varying electrical signal that is derived from a time amplitude varying electro-magnetic energy signal that strikes said photodetector,

means receiving said time varying electrical signal for establishing substantially a short circuit across said capacitor in response to an electro-magnetic energy signal striking the photodetector, whereby said storage capacitor is immediately discharged upon receipt of an electro-magnetic energy pulse of adequate intensity that strikes the photodetector, and

means monitoring the voltage level of said storage capacitor for initiating an alarm when said capacitor voltage increases to a predetermined level.

9. The electronic circuit according to claim 8 wherein said capacitor short circuit includes a three terminal semi-conductor device with two of its terminals being connected directly across said capacitor without any additional electronic components connected in series therewith, a third terminal of said semiconductor device being a control terminal which switches on and off an electrical path between its said two terminals in response to the voltage at said third terminal, said third terminal being connected to receive said time varying electrical signal from the photodetector.

10. The electronic circuit according to claim 8 which additionally comprises means for generating an optical beam adapted to be directed a distance onto said photodetector, said beam generating means including means for forming an optical beam of periodic single polarity pulses with substantially zero intensity between pulses.

11. The circuit of claim 8 which additionally comprises means controlling said alarm for maintaining the alarm in an active state for a predetermined time after initiated by said alarm initiating means.

12. The circuit of claim 8 wherein said photodetector is a phototransistor biased in a non-linear gain region with a signal output many times an output when said phototransistor is operated as a photodiode.

13. An electronic circuit for detecting the existence of an object in a space through which a time varying optical signal beam passes, comprising:

a voltage supply source,

a first storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor,

means including a photodetector for producing a time varying electrical signal that is derived from a time varying optical signal that strikes said photodetector,

means responsive to the light detector time varying output signal for establishing a discharge path across said first capacitor that discharges said first capacitor faster than said resistor permits the first capacitor to charge,

means monitoring the voltage level of said storage capacitor for instantaneously charging a second storage capacitor when the voltage across said first storage capacitor reaches a predetermined threshold level,

means for slowly discharging said second capacitor,

means monitoring the voltage across said second storage capacitor for emitting a control signal that is initiated when said second capacitor storage means charges said second capacitor and which is terminated when the second capacitor discharges to a predetermined voltage level, and

means receiving said control signal for generating an alarm in response thereto, whereby the alarm is operative for a length of time after each charging of the second storage capacitor that is determined by the discharge time constant of the second storage capacitor and the predetermined voltage level which causes the control signal to be terminated.

14. Apparatus for remotely indicating the existence of a desired condition, comprising:

means forming a unit of apparatus for detecting the existence of said desired condition,

means forming a second distinct unit of apparatus for indicating the existence of said desired condition,

means including first, second and third connectors on each of said detector unit and indicating unit for interconnecting the units by first, second and third conductors, respectively,

an electronic power supply as part of said indicating unit and connected to develop a supply voltage between said second and third connectors of said indicating unit,

said detecting unit including an output semiconductor device that is normally held in a conductive state across the first and second connectors of the detecting unit but is connected to be switched to a non-conductive state when said desired condition is detected,

a load element within said indicating unit extending between its said first and third connectors of said indicating unit, and

means as part of the indicating unit for operating an indicating device when a voltage across said load element increases significantly, whereby when either said first, second or third conductors are cut the indicating device will give this warning without any additional elements necessary between the two units.

15. The system according to claim 14 wherein said detecting unit includes a photodetector and means for focusing an optical beam onto said photodetector, thereby to provide an intruder alarm system which cannot be disabled by an intruder without setting off the indicating device. 

1. An electronic circuit for detecting the existence of an object in a space through which an electro-magnetic radiation beam passes, comprising: a phototransistor having emitter, collector and base electrodes, means for directing electro-magnetic radiation onto said phototransistor, whereby said phototransistor may be exposed to said electro-magnetic radiation beam, said collector and emitter electrodes being connected in series with a load resistor across a two terminal voltage supply, means connected between said voltage supply and said base electrode of the phototransistor for providing a bias that increases the zero radiation signal collector current at least several times a normal collector current that exists when the base terminal remains unconnected, said bias further set for said phototransistor to operate in a non-linear gain region with A collector bias current below that which causes the phototransistor to operate with linear gain characteristics, an object indicating device having two states, one state indicating an object present and another state indicating an object not to be present, and means responsive to the voltage drop across said load resistor for driving said indicating device so that it is in one of its said states when said phototransistor is receiving electromagnetic energy and another state when that electromagentic energy has been interrupted for a selected period of time.
 2. An electronic circuit according to claim 1 wherein said indicating device driving means comprises a non-linear semi-conductor amplifier element that is biased to operate in a range of its non-linearity with its steady state current at a very low value, whereby said amplifier draws very little current when no signal is impressed thereon.
 3. The electronic circuit according to claim 1 wherein said indicating device means comprises: a direct current voltage supply source, a storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor, means receiving said voltage across said load resistor for establishing substantially a short circuit across said capacitor means in response to electro-magnetic energy striking the phototransistor, whereby said storage capacitor is immediately discharged by a pulse of electro-magnetic energy of adequate intensity striking the phototransistor, and means monitoring the voltage level of said storage capacitor for initiating a change of state of said intruder indicating device when said capacitor voltage increases to a predetermined level.
 4. The electronic circuit according to claim 3 wherein the object detecting device state change initiating means includes means for instantaneously charging a second storage capacitor simultaneously with a change of state of such indicating device, means for slowly discharging said second capacitor, means monitoring the voltage across said second storage capacitor for reversing the change of state of said indicating device when the second capacitor discharges down to a predetermined voltage level.
 5. The electronic circuit according to claim 1 which additionally comprises a pulsed source of electro-magnetic radiation adapted to form said radiation into a beam that may be directed at said phototransistor, said pulses all having the same polarity with substantially no electro-magnetic energy intensity in the period between pulses.
 6. The circuit of claim 1, wherein said bias providing means is a resistance connected between said base electrode and one of the terminals of said voltage supply, and further wherein said circuit additionally comprises a capacitance connected between said base electrode and the other of said voltage terminals.
 7. The circuit of claim 1, wherein said indicating device driving means includes a capacitive coupling between said indicating device and a point of connection of said load resistor and an electrode of said phototransistor.
 8. An electronic circuit for detecting the existence of an object in a space through which a time amplitude varying electro-magentic energy beam passes, comprising: a voltage supply source, a storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor, means including a photodetector for producing a time varying electrical signal that is derived from a time amplitude varying electro-magnetic energy signal that strikes said photodetector, means receiving said time varying electrical signal for establishing substantially a short circuit across said capacitor in response to an electro-magnetic energy signal Striking the photodetector, whereby said storage capacitor is immediately discharged upon receipt of an electro-magnetic energy pulse of adequate intensity that strikes the photodetector, and means monitoring the voltage level of said storage capacitor for initiating an alarm when said capacitor voltage increases to a predetermined level.
 9. The electronic circuit according to claim 8 wherein said capacitor short circuit includes a three terminal semi-conductor device with two of its terminals being connected directly across said capacitor without any additional electronic components connected in series therewith, a third terminal of said semi-conductor device being a control terminal which switches on and off an electrical path between its said two terminals in response to the voltage at said third terminal, said third terminal being connected to receive said time varying electrical signal from the photodetector.
 10. The electronic circuit according to claim 8 which additionally comprises means for generating an optical beam adapted to be directed a distance onto said photodetector, said beam generating means including means for forming an optical beam of periodic single polarity pulses with substantially zero intensity between pulses.
 11. The circuit of claim 8 which additionally comprises means controlling said alarm for maintaining the alarm in an active state for a predetermined time after initiated by said alarm initiating means.
 12. The circuit of claim 8 wherein said photodetector is a phototransistor biased in a non-linear gain region with a signal output many times an output when said phototransistor is operated as a photodiode.
 13. An electronic circuit for detecting the existence of an object in a space through which a time varying optical signal beam passes, comprising: a voltage supply source, a first storage capacitor connected to said voltage source through a series connected resistor, whereby said voltage supply source normally charges said capacitor at a rate determined by the values of said capacitor and resistor, means including a photodetector for producing a time varying electrical signal that is derived from a time varying optical signal that strikes said photodetector, means responsive to the light detector time varying output signal for establishing a discharge path across said first capacitor that discharges said first capacitor faster than said resistor permits the first capacitor to charge, means monitoring the voltage level of said storage capacitor for instantaneously charging a second storage capacitor when the voltage across said first storage capacitor reaches a predetermined threshold level, means for slowly discharging said second capacitor, means monitoring the voltage across said second storage capacitor for emitting a control signal that is initiated when said second capacitor storage means charges said second capacitor and which is terminated when the second capacitor discharges to a predetermined voltage level, and means receiving said control signal for generating an alarm in response thereto, whereby the alarm is operative for a length of time after each charging of the second storage capacitor that is determined by the discharge time constant of the second storage capacitor and the predetermined voltage level which causes the control signal to be terminated.
 14. Apparatus for remotely indicating the existence of a desired condition, comprising: means forming a unit of apparatus for detecting the existence of said desired condition, means forming a second distinct unit of apparatus for indicating the existence of said desired condition, means including first, second and third connectors on each of said detector unit and indicating unit for interconnecting the units by first, second and third conductors, respectively, an electronic power supply as part of said indicating unit and connected to develop a supply voltage between said second anD third connectors of said indicating unit, said detecting unit including an output semi-conductor device that is normally held in a conductive state across the first and second connectors of the detecting unit but is connected to be switched to a non-conductive state when said desired condition is detected, a load element within said indicating unit extending between its said first and third connectors of said indicating unit, and means as part of the indicating unit for operating an indicating device when a voltage across said load element increases significantly, whereby when either said first, second or third conductors are cut the indicating device will give this warning without any additional elements necessary between the two units.
 15. The system according to claim 14 wherein said detecting unit includes a photodetector and means for focusing an optical beam onto said photodetector, thereby to provide an intruder alarm system which cannot be disabled by an intruder without setting off the indicating device. 